Image display devices using cathode ray tubes are widely used for many applications, such as the representation of visual images ranging from the realistic scenes of entertainment television to the complex graphics and coded data generated by information processing systems, such as computers and the like. High definition television displays are one example of the recent trend towards utilizing cathode ray tubes. To ensure satisfactory performance, it is necessary that an electron beam be properly focused over the entire fluorescent surface of the cathode ray tube. Furthermore, in projection television systems, it is necessary to produce proper focus uniformity, because the enlarged image is projected onto the remote screen.
Within the cathode ray tube, therefore, it has become desirable to change the conventional focus condition, because the distance from the deflection point of the beam to the periphery of the surface is longer than the distance from the deflection point to the center of the surface. The conventional method of changing the focus condition in order to focus the electron beam over the entire surface is to apply a parabolic compensation signal, synchronized to a horizontal and a vertical synchronizing signal, to a focus coil or a focus electrode of the cathode ray tube. However, this method is insufficient to meet the severe focusing requirements of high definition television and projection systems because of the following problems: 1) the processing measure error of the cathode ray tube; 2) the dimension and location errors of several electrodes; and 3) the aberration of the electron beam spot.
Thus, there exists a need in the art for a dynamic focusing device that overcomes the limitations of the prior art and achieves the important advantages of providing proper focus uniformity over the entire surface of a cathode ray tube, so as to meet the stringent focusing demands of high definition televisions and projection systems.